Electrophotographic compositions and imaging processes are well known. In these processes, an electrophotographic element having a layer containing a photoconductor is electrostatically charged and then imagewise exposed to form a latent electrostatic image. The latent electrostatic image is subsequently developed with a toner composition. Electrophotographic elements and processes are disclosed, for example, in U.S. Pat. Nos. 3,141,770 to Davis et al., 3,554,745 to Van Allen, 3,577,235 to Contois, 3,615,414 to Light et al., 4,442,193 to Chen et al., 4,421,837 to Hiroshi et al., and 4,468,444 to Contois. Unfortunately, with any electrophotographic element, it is always necessary to charge electrostatically and imagewise expose the charged element for each copy being made.
Multiple copies have been made from a single exposure using photoelectrographic elements in which a persistent differential conductivity pattern is created between exposed and unexposed portions. This allows for subsequent use of the element in printing multiple copies from a single exposure with only multiple charging, developing, transferring, and cleaning steps. This is different from electrophotographic imaging techniques where the electrophotographic element must generally be charged electrostatically followed by imagewise exposure for each copy produced.
Photoelectrographic masters are ideal for use in xeroprinting or multiple color proofing, because multiple high-quality prints can be produced rapidly in view of the need for only a single exposure. This is especially useful in making color images.
One type of master, disclosed in U.S. Pat. No. 4,818,660 to Blauchet-Fincher et al. and U.S. Pat. No. 4,859,551 to Kempf, is prepared by coating a photohardenable layer on an electrically conductive substrate and exposing the layer imagewise to light. Exposed portions of the photohardenable layer harden and become nonconductive, while the unexposed parts of the layer remain unhardened and conductive. When the master is electrostatically charged and developed by applying a toner of opposite charge, the toner adheres to exposed areas. Such films, however, are difficult to handle due to the tackiness of unhardened polymer.
Photoelectrographic master elements generally have a conductive layer in electrical contact with a film layer. When exposed to ultraviolet radiation, photochemically-generated charges form in the film, making the film conductive, while unexposed areas of the film remain insulating. When the element is charged, charges at the surface of the element and at the interface between the film and the conductive layers are neutralized where exposure has occurred. Unexposed areas, however, are charged and then developed with toner. The toned image is transferred to a receptor sheet. In U.S. Pat. No. 4,661,429 to Molaire et al., the film layer includes an aromatic onium salt or a 6-substituted-2,4-bis (trichloromethyl)-5-triazine acid photogenerator, an insulating binder, and, optionally, a sensitizer.
Photoelectrographic elements capable of exposure with near-infrared radiation (having wavelengths of 650 to 1000 nm) have also been developed. Such elements are particularly desirable because radiation in this part of the spectrum is emitted by laser diodes which are relatively inexpensive and consume little energy.
U.S. Pat. No. 3,909,254 to Tamai discloses a photoelectrographic master element, containing an organic photoconductor and a polymeric resin, which is exposed with a laser. The organic photoconductor of this element operates in a conventional fashion by normally being non-conductive and achieving conductivity when exposed.
U.S. Pat. No. 4,124,286 to Barasch discloses a xerographic printing process in which a first source of information is imaged on a photoconductive medium, capable of achieving persistent conductivity, to form a conductive representative image. The conductive image is transferred to another photoconductor on which a complimentary source of information is imaged with a laser.
U.S. Pat. No. 4,047,945 to Pfister relates to a xeroprinting master element consisting essentially of a conductive base member, a non-persistent photoconductive insulating layer, a persistent photoconductive insulating layer containing an acid sensitive charge transfer complex, and a dielectric layer. The process of utilizing the element comprises: charging, blanket exposing the non-persistent photoconductive insulating layer without activating the persistent photoconductive insulating layer, and developing after field collapse across the non-persistent photoconductive layer. When blanket exposed, the interface between the insulating layers provides a barrier to charge injection in non-imaged areas.
These elements, however, have not received wide-spread acceptance, because they employ a complicated construction and are utilized in a complex process.